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Full text of "Mathematical And Physical Papers - Iii"

344           ON  THE  CHANGE  OF  REFRANGIBILITY  OF  LIGHT.
the same time; or, if it be desired to see the latter more distinctly, it will be sufficient to attach a fragment of paper to the mineral or other substance, placing it so that the image of the slit is formed partly on the paper and partly on the substance to be examined. I have frequently found this mode of observation convenient in examining the absorption of light by opaque substances. The manner in which the absorption of the medium comes into play in this case will be considered in greater detail further on (see Art. 176).
143.    When green uranite was examined in this manner, it showed a very remarkable system of dark bands of absorption. These bands were seven in number, or at any rate six, and were arranged with all the regularity of bands of interference.  The first was situated at about b$F, the second at F\ the middle of the sixth fell a very little short of 0; the third, fourth and fifth were arranged at regular intervals between the second and sixth; the seventh was situated about as far beyond the sixth as the sixth beyond the fifth.    The spectrum was so faint in the region of the seventh band as to leave some slight doubts respecting its existence.     There would not have been light enough to see bands further on.
144.    Uranite is highly lamellar in its structure, from whence it is otherwise called uran-rnica.   The reader may perhaps suppose that the dark bands described in the last paragraph were bauds of interference, which I had mistaken for bands of absorption, and that they were really of the nature of Newton's rinses, or more exactly of the bands seen in an experiment due to the Baron von Wrede.    There may, it will perhaps bo said, have been a fissure parallel to the first surface, so as to separate a, thin plate; and the interference of the two streams of light reflected respectively on the upper and under surface of this plate may have produced the bands observed.    But various phenomena attending these bands are irreconcilable with such a supposition.     Towards the edges of the crystal, where flaws did in fact exist, bands of the same nature as  Von  Wrede's   were   actually   observed.      But   these   had   an appearance totally different from that of the others.    The dark bands of the interference system were more intensely black and better defined than those of the other system, and  were  very